A variety of optical measuring systems must be calibrated prior to use. One example of such a system is an oximetry system which must provide accurate in vivo measurements of mixed venous oxygen saturation. A system of this type includes an optical thermodilution catheter, a light source for supplying light at selected wavelengths to the optical catheter, and an optical measuring device for measuring the intensity of the light signal received from the optical catheter. Any of these components can introduce variations into the system, and thus, the entire system must be calibrated.
The oximetry system can be calibrated using in vivo techniques. According to this approach, an oximeter catheter is inserted into the vascular system of a patient, and light intensity measurements of the blood are made. A blood sample is also drawn from the patient, and the degree of oxygen saturation of the sample is accurately measured in a laboratory. These two measurements are then appropriately correlated to calibrate the oximetry system. This technique requires that a blood sample be drawn, and this can introduce error due to sampling techniques and due to time lags, such as the time lag between the sample time and the completion of the laboratory analysis of the sample. In addition, this technique is very time consuming.
The optical system can also be calibrated using in vitro techniques. One such technique requires that a reference element be brought into optical contact with the end of an optical catheter while the catheter is clamped in position. The clamping of the optical catheter, particularly in the region of the balloon, may damage the balloon. In addition, a mechanism for moving the reference element is required. This technique also requires that the reference element be compliant at the surface which contacts the end of the catheter. Such a device is shown in Shaw et al U.S. Pat. No. 4,322,164.
It is also known to employ an optically open system as shown in Polyani et al U.S. Pat. No. 4,050,450. However, being optically open, this system is not immune to ambient light.